1. Field of the Invention
The present invention relates generally to hybrid motor vehicles and, more particularly, to a battery control system for controlling the state of charge of a hybrid vehicle battery.
2. Background of the Invention
Motor vehicle manufacturers are actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrains equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, “hybrid” vehicles, which are equipped with an internal combustion engine and an electric motor that can be operated independently or in combination with the internal combustion engine to provide motive power to the vehicle, offer a compromise between traditional internal combustion engine powered vehicles and full electric powered vehicles.
A hybrid vehicle is typically equipped with a battery (a main battery) that provides electrical power to the hybrid driving motor and, in turn, is charged by a generator (typically the hybrid drive motor operating as a generator to generate electrical energy from a mechanical input). A battery charge level is monitored according to an index known as the battery State of Charge, or SOC. The SOC is defined by a ratio of the amount of residual charge remaining in a battery relative to its full charge capacity. Presently, a battery's SOC is generally measured using a combination of a measurement method utilizing a correlation between SOC and a battery's voltage-current characteristics at the time of charging (or discharging) and a measurement method utilizing an accumulation of charged and discharged amounts.
In hybrid vehicles, a battery control system typically controls charging and discharging of a battery based on the SOC. Specifically, for charging and discharging control, SOC values are typically divided into at least three ranges, namely, a charging prohibited range, a discharging prohibited range, and a normally charged range. In a charging prohibited range (e.g., SOC about 70 to 100%), further charging may be prohibited to leave room for energy capture through regenerative braking and to avoid excessive charging that could damage the battery. In a discharging prohibited range (e.g., SOC about 0 to 30%), further discharging may be prohibited to prevent excessive discharging that could compromise normal operation of the hybrid powertrain system. In a normally charged range (e.g., SOC about 30 and 70%), excessive charging or discharging are both unlikely and, therefore, charging and discharging are both allowed.
The preceding SOC strategy, while satisfactory for operating a passenger vehicle, may not be suitable for hybrid vehicles that operate the hybrid powertrain system to produce on-board of off-board power. For example, hybrid passenger vehicles use energy from the battery to enhance dynamic performance, convert kinetic energy into electric energy using regenerative braking, and store the converted energy in a battery to improve power consumption. In addition to these features, hybrid utility vehicles, such as utility trucks, may be operated to produce off-board electric or hydraulic power by operating the electric motor to produce electrical energy to drive a hydraulic pump, or use the electrical energy stored in the main battery to power on-board or off-board electrical equipment. In the latter example, it may be desirable to modify the SOC level during certain vehicle operating states (e.g., stationary vehicle supplying off-board electrical power) to maximize the energy available in the main battery and to minimize the need to activate the internal combustion to recharge the main battery or supplement the power provided thereby.